1. Field of the Invention
The present invention relates to an acrylic polymer composition wherein substantially no solvent is contained, an acrylic pressure sensitive adhesive tape prepared with the use thereof and processes for producing these.
2. Description of the Prior Art
Acrylic monomers have excellent polymerizability, and can be polymerized by various reaction systems including the solution polymerization, suspension polymerization, emulsion polymerization and bulk polymerization techniques.
For example, the following processes have been proposed for production of acrylic polymers. One process comprises heating a mixture of an acrylic monomer and a mercaptan at a temperature ranging from 20 to 200xc2x0 C. in the presence of oxygen to there by effect a bulk polymerization thereof (see Japanese Patent Publication No. 50(1975)-401). Another process comprises polymerizing a mixture of an acrylic monomer and a mercaptan wherein substantially no initiator is contained in a nitrogen atmosphere (see Japanese Patent No. 258,251). A further process comprises carrying out polymerization with the use of an extrusion type barrel apparatus, in place of a batch reaction vessel, at high temperatures (near 150xc2x0 C.) (see Japanese Patent Publication No. 2(1990)-55448). Still a further process comprises irradiating a batch reaction vessel with UV light through an optical fiber and carrying out polymerization by pulse irradiation of UV light (see Japanese Patent Laid-open Publication No. 7(1995)-330815). Yet still a further process comprises carrying out a UV bulk polymerization by stepwise changing the reaction temperature while irradiating a batch reaction vessel with UV light (see Japanese Patent Laid-open Publication No. 11(1999)-49811).
However, when it is intended to react acrylic monomers in the presence of a heat decomposable polymerization initiator in conventional batch reaction vessels of industrial scale, because of the high reactivity of acrylic monomers, the heat generation in the reactor (reaction vessels) is so intense that it is difficult to expel the heat of reaction toward outside of the reaction system. Therefore, it has been impracticable to accomplish a bulk polymerization of acrylic monomer in the presence of a heat decomposable polymerization initiator in reaction vessels while effectively controlling the reaction.
The reaction using a barrel apparatus has a drawback in that it is needed to set the reaction temperature so as to fall within a high temperature region with the result that, in accordance with the lowering of precision of temperature control, the molecular weight distribution of obtained polymer will be broadened and the molecular weight of obtained polymer will become polydisperse.
With respect to the barrel apparatus including UV irradiation means, temperature control is difficult, so that a reaction control of high precision cannot be effected. Further, with respect to the batch reaction vessel equipped with UV irradiation means, the cost on cooling facilities for controlling the heat generation accompanied with scale increase is so large that it is not suitable for mass production of an acrylic polymer by the use of existing facilities.
On the other hand, as aforementioned, the bulk polymerization technique is known as a method of polymerizing an acrylic monomer. In the bulk polymerization, the produced polymer does not contain solvents and does not contain surfactants and the like. Thus, in the bulk polymerization, an operation of separating solvents from the produced polymer is not needed. The produced polymer does not contain surfactants and other matters which are likely to cause deterioration of water resistance and other properties. For these reasons, the bulk polymerization, when assessed only from the viewpoint of reaction mode, provides a preferable reaction mode.
However, in the bulk polymerization wherein use is made of a thermal polymerization initiator, it is extremely difficult to control the thermal polymerization reaction because of the high reactivity of monomers used, so that the runaway of polymerization reaction is likely to occur. The runaway reaction means a phenomenon such that the reaction can no longer be controlled to thereby cause violent advance of the reaction. That is, the runaway reaction is extremely dangerous because of a rapid change of the state of components charged in the reactor, for example, a rapid rise of the reaction temperature. Moreover, the molecular weight distribution of produced polymer tends to be broad, and the molecular weight of obtained polymer tends to be low.
With respect to the bulk polymerization technique wherein an acrylic monomer is used, Japanese Patent Laid-open Publication No. 53(1978)-2589 discloses a process for producing a thermosetting acrylic resin, comprising polymerizing a mixture or syrup of a (meth)acrylic ester and a crosslinking monomer, wherein first a prepolymer with a polymerization degree of 60% or more is produced at 150xc2x0 C. or below in a vessel type reactor, the prepolymer is taken out from the vessel type reactor, and further polymerization of the prepolymer is conducted through multi-stage polymerization steps set for 10 to 60% polymerization degree differences. In the Example section of the published specification, azobisisobutyronitrile, tert-butyl peroxylaurate or the like is used in an amount of about 0.01 to 0.3 part by weight per 100 parts by weight of monomer. The 10-hr half-life temperature of azobisisobutyronitrile is 66xc2x0 C. The 10-hr half-life temperature of tert-butyl peroxylaurate is 98.3xc2x0 C. If these thermal polymerization initiators of high 10-hr half-life temperatures are used for the above monomers in an amount of about 0.01 to 0.3 part by weight, the temperature of reaction system would rapidly rise simultaneously with the initiation of reaction. Thus, the reaction would run away unless a cooling unit of high performance is provided. Therefore, in the invention described in the published specification, it is inevitably needed to advance multi-stage polymerization reaction while effecting satisfactory cooling so as to prevent the runaway of reaction at each stage with the use of a reactor equipped with a cooling unit of satisfactory cooling capacity. Consequently, in the process described in the published specification, a high-performance equipment must be installed for cooling the reaction system.
Furthermore, Japanese Patent Laid-open Publication No. 58(1983)-87171 discloses a process for producing an acrylic pressure sensitive adhesive of 100 thousand to 600 thousand of weight average molecular weight, which comprises the first stage wherein 0.00005 to 0.5 part by weight of a thermal polymerization initiator whose half-life period is in the range of 0.1 to 1000 hr at 70xc2x0 C. and in the range of 0.1 to 5 hr at the polymerization initiation temperature is mixed with 100 parts by weight of an acrylic monomer and the acrylic monomer is polymerized at 40 to 120xc2x0 C., and the second stage wherein 0.0001 to 1 part by weight of a thermal polymerization initiator whose half-life period is longer than 1000 hr at 70xc2x0 C. and 2 hr or longer at the polymerization initiation temperature is added and polymerization is carried out at a temperature which is higher than that of the first stage but in the range of 100 to 200xc2x0 C. In the published specification, as examples of polymerization initiators used in the invention described therein, there are mentioned organic peroxides such as acetyl peroxide, lauroyl peroxide, benzoyl peroxide, diisopropyl peroxide, di-2-ethylhexyl peroxydicarbonate, tert-butyl peroxy(2-ethyl hexanoate), tert-butyl peroxylaurate and tert-butyl peroxyacetate; and azo compounds such as azobisisobutyronitrile and 2,2xe2x80x2-azobis(2,4-dimethylvaleronitrile).
The polymerization initiators used in the invention described in this published specification are thermal polymerization initiators, and the 10-hr half-life temperatures thereof are in the range of 43 to 102xc2x0 C. Upon reviewing the thermal polymerization initiators used in the invention described in this published specification, it is noted that there is no technical idea of selectively employing a thermal polymerization initiator of specified half-life temperature and that customary thermal polymerization reaction initiators are used. Rather, the description as to polymerization initiators in the invention described in this published specification is directed to the amount of polymerization initiator used, and it is described therein that an extremely small amount of thermal polymerization initiator can be used. Even if it is intended to control the reaction temperature within the range of 40 to 120xc2x0 C. with the use of the above thermal polymerization initiators, the heat generation is highly dependent on the activity of employed reaction initiator. For example, in the use of tert-butyl peroxide, benzoyl peroxide or the like as described in the Example section, the temperature of reaction system would sharply rise simultaneously with the initiation of reaction. Therefore, there would occur such a problem that, for suppressing the heat generation, a cooling unit of extremely high performance must be provided.
Still further, Japanese Patent No. 2,752,458 discloses a process for producing a methacrylic polymer, comprising the steps of charging a monomer mixture whose principal component is methyl methacrylate in a complete mixing type reactor, adjusting dissolving oxygen in the monomer mixture to 1 ppm or less, and, in the presence of a thermal polymerization initiator whose half-life period at polymerization temperature is in the range of 0.5 to 120 sec, carrying out polymerization at a temperature within the range of 130 to 160xc2x0 C. under agitation by a specified agitating power with the average residence time set so that the average residence time and half-life period of radical polymerization initiator fall within specified ranges to thereby attain a monomer conversion of 45 to 70%.
Radical polymerization initiators practically used in the Example section of the published specification are, for example, 2,2-azobisisobutyronitrile, tert-butyl peroxyisobutyrate and lauroyl peroxide. The 10-hr half-life temperatures there of exceed 41xc2x0 C. Therefore, for example, the Example section and FIG. 2 show the use of a cooling unit of extremely high performance for suppressing runaway reactions, for example, the use of a heat exchanger with the use of, for example, xe2x88x925xc2x0 C. of refrigerant as a cooling unit.
As apparent from the above, in the conventional bulk polymerization processes, there is no technical idea of selecting a polymerization initiator for use, and the technique of carrying the generated heat to the outside of the reaction system by means of a cooling unit of high performance to thereby suppress the runaway reaction has been employed. Thus, these processes have a drawback in that the cooling unit is extremely expensive. Further, in the industrial-scale production of an acrylic polymer wherein a homogeneous advance of reaction is difficult, even the use of a cooling unit of quite high performance would not resolve the extreme difficulty in uniformly cooling the entirety of reaction tank. Consequently, if a reaction runaway occurs at part of the inside of the reactor, there is the danger of extending of the runaway reaction to the whole of reaction system. Thus, even if a reaction ensures stable advance on an experimental level, a straight scale-up thereof to an industrial process might be inappropriate.
Apart from the above, the acrylic polymers which can be produced by these various polymerization processes find a variety of applications. In particular, the acrylic polymers are widely used in pressure sensitive adhesives. Such pressure sensitive adhesives have been incorporated with tackifiers whose representative examples are rhodinic acid derivatives.
For example, with respect to an acrylic pressure sensitive adhesive, (meth)acrylic polymers themselves have tackiness and, even if no tackifier is added, can provide a pressure sensitive adhesive of excellent thermal stability and weather resistance. However, this pressure sensitive adhesive is inferior to pressure sensitive adhesives incorporated with tackifiers in adherence at room temperature and adherence to a low-energy surface such as a surface of polyolefin, automobile coating or the like. Thus, the addition of a tackifier would impart advantageous adhesive properties.
However, the improvement of adhesive properties realized by the addition of a tackifier is not necessarily satisfactory. Further, the addition of a tackifier to acrylic pressure sensitive adhesives would cause the following problems. That is, the addition of common tackifiers whose representative examples are rhodinic acid derivatives to acrylic pressure sensitive adhesives would often cause deteriorations of transparency and weather resistance. Further, when such tackifiers are present at the time of bulk polymerization reaction, the tackifiers would act as a chain transfer agent or a reaction terminating agent, depending on the structure thereof, so that there is the danger of inviting the inhibition or retardation of polymerization reaction.
Also, it is known to add an acrylic polymer as a tackifier to a pressure sensitive adhesive. For example, Japanese Patent Laid-open Publication No. 54(1979)-3136 discloses a pressure sensitive adhesive containing both an acrylic polymer and a tackifier. The tackifier used therein is one obtained by a solution polymerization of a mixture of a vinyl aromatic compound and a (meth)acrylic ester, which has a number average molecular weight of 500 to 3000 and a softening point of 40xc2x0 C. or below. Further, Japanese Patent Laid-open Publication No. 1(1989)-139665 discloses the invention of an adhesive composition containing a polymeric additive having a number average molecular weight of 35,000 or less and a softening point of 40xc2x0 C. or above, which polymeric additive is obtained by polymerizing a (meth)acrylate having an alkyl or a cycloalkyl group of 1 to 20 carbon atoms and a free-radical compatible olefinic acid (for example, acrylic acid or the like) optionally together with another ethylenic unsaturated monomer. This published specification describes that the polymeric additive can be produced by any of the emulsion polymerization, suspension polymerization, solution polymerization and bulk polymerization. It is further described in the published specification that xe2x80x9cThis polymeric additive is added to the adhesive composition by known methods, for example, mixing and blending, and the added matter is homogeneously contained in the adhesive composition. The additive is preferably added to the adhesive composition in the form of an emulsion, or a solute emulsified in a mixture of an aqueous solvent and an organic solvent.xe2x80x9d
That is, it is described that the polymeric additive described in this published specification is produced by the emulsion polymerization or solution polymerization, and that the polymeric additive in a form emulsified or dissolved in a solvent is added to the adhesive prepared by the emulsion polymerization or solution polymerization.
A reason for the use of the polymeric additive and adhesive obtained by the emulsion polymerization or solution polymerization without exception in these published specifications would be that, although that an acrylic monomer can be polymerized without the use of a reaction solvent (bulk polymerization) has been known, it is extremely difficult, as mentioned above, to control the reaction in the polymerization of an acrylic monomer performed without the use of a reaction solvent, namely, the bulk polymerization, thereby failing to produce a polymer selectively with specified properties. Another reason would be nothing but that a homogeneous composition can be easily obtained by mixing the polymeric additive and adhesive produced by the solution polymerization or emulsion polymerization with a solvent or a dispersion medium.
However, when bonding is conducted with the use of such a pressure sensitive adhesive composition containing a solvent or an emulsion type pressure sensitive adhesive composition, an operation of removing the solvent or dispersion medium must be needed. For example, in the use of water whose latent heat of evaporation is large, a prolonged drying operation must be effected. Further, when an organic solvent is used, not only a device for trapping an evaporated organic solvent must be installed but also there occurs such a problem that the influence on environment must be apprehended.
On the other hand, processes for producing a pressure sensitive adhesive without the use of any solvent or dispersion medium are described in Japanese Patent Laid-open Publication Nos. 50(1975)-87129 and 50(1975)-102635. These published specifications disclose a process for producing a pressure sensitive adhesive tape, comprising coating a support sheet with a mixture obtained by mixing a mixture of a (meth)acrylic acid alkyl ester and a vinyl monomer having a specified polar group with a (co)polymer thereof and a polymerization initiator and subsequently carrying out thermal polymerization of the mixture on the support sheet. In the invention of Japanese Patent Laid-open Publication No. 50(1975)-87129, use is made of a redox type polymerization initiator because, with the use of common polymerization initiators, polymerization cannot smoothly advance due to the polymerization inhibiting action of oxygen. In the invention of Japanese Patent Laid-open Publication No. 50(1975)-102635, thermal polymerization reaction is carried out while avoiding the contact with oxygen by overlaying the raw material pressure sensitive adhesive composition after coating with a release sheet or belt.
As aforementioned, without the use of an organic solvent, a tape with pressure sensitive adhesive property can be produced by incorporating a (meth)acrylic acid alkyl ester as a solvent with an acrylic (co)polymer.
However, the processes disclosed in these published specifications have a drawback in that, because the monomer used as a solvent is polymerized by thermal polymerization, there is constantly the danger of reaction termination by the contact with oxygen at surface portion of the coating layer where the possibility of contact with oxygen in air is high, and that thus the uniform advance of polymerization in the direction of layer thickness is difficult. Further, the monomer used therein has a composition identical with or close to that of the (co)polymer as a solute because the monomer must dissolve the (co)polymer. Therefore, the pressure sensitive adhesive layer of the pressure sensitive adhesive tapes obtained by the processes described in these published specifications is constituted of (co)polymers of substantially the same composition.
Pressure sensitive adhesive layers constituted of such (co)polymers of substantially a single composition would not exhibit satisfactory bond strength to an adherend whose bonding is considered to be difficult, such as a polyolefin.
Apart from the above, the acrylic pressure sensitive adhesive is often used in the form of a pressure sensitive adhesive tape obtained by coating a flexible support constituted of, for example, a paper or plastic with the acrylic pressure sensitive adhesive.
With respect to the acrylic pressure sensitive adhesive for use in the above acrylic pressure sensitive adhesive tape, the viscosity thereof must be low to a certain level for enabling coating to obtain the acrylic pressure sensitive adhesive layer. Accordingly, the pressure sensitive adhesive obtained by polymerization using a solvent or dispersion medium, such as the solution polymerization or emulsion polymerization, is used as the above pressure sensitive adhesive for use in the acrylic pressure sensitive adhesive tape. That is, the pressure sensitive adhesive tape is produced by coating a support with the pressure sensitive adhesive produced by the aforementioned processes together with the solvent or dispersion medium and expelling the solvent or dispersion medium.
However, the pressure sensitive adhesive tape from the pressure sensitive adhesive obtained by the above polymerization processes has a drawback in that not only is high energy required for drying but also the pressure sensitive adhesive tape has a problem of smell attributed to residual solvent. Further, the pressure sensitive adhesive tape obtained through water-based polymerization often fails to have satisfactory water resistance. As aforementioned, the bulk polymerization is known as a polymerization method wherein neither solvent nor dispersion medium is used. However, in the bulk polymerization, not only controlling of the reaction is difficult to thereby cause it extremely difficult to obtain a pressure sensitive adhesive tape of stable properties but also the polymer obtained by the bulk polymerization generally has such a high viscosity that coating of supports is extremely difficult. Therefore, in the use of a pressure sensitive adhesive not containing any solvent or dispersion medium, for example, the hot melt technique wherein the viscosity of pressure sensitive adhesive used in coating is lowered by heating is employed. It is also known to produce a pressure sensitive adhesive tape by polymerizing a monomer applied on a support with, for example, the irradiation of ultraviolet light (see Japanese Patent Laid-open Publication Nos. 5(1993)-5014 and 9(1997)-111195).
As described with respect to this process, however, polymerization requires a prolonged period of time in a single-photopolymerization-step process, so that, in the mass production of, for example, a pressure sensitive adhesive tape, producing a pressure sensitive adhesive from a monomer by a single-photopolymerization-step process is extremely disadvantageous from the viewpoint of cost. Therefore, a pressure sensitive adhesive tape is produced through a plurality of photopolymerization steps. In the initial polymerization of such a monomer, a prepolymerization through photopolymerization reaction is generally employed because, as described above, reaction control is extremely difficult if thermal polymerization is employed in the partial polymerization of monomer.
The present invention is based on finding of a method capable of stably advancing reaction without runaway thereof in the bulk polymerization process. Thus, it is an object of the present invention to provide an acrylic pressure sensitive adhesive tape wherein use is made of a partial polymerizate obtained by the method, and to provide a process for producing such a pressure sensitive adhesive tape.
In particular, it is an object of the present invention to provide a process for stably producing an acrylic pressure sensitive adhesive tape having excellent adhesive properties, and to provide an acrylic pressure sensitive adhesive tape having excellent adhesive properties, especially properties such as thermal stability and water resistance.
It is another object of the present invention to provide a novel acrylic pressure sensitive adhesive composition which contains substantially no solvent.
It is a further object of the present invention to provide an acrylic pressure sensitive adhesive composition which enables the production of an adhesive capable of exhibiting an extremely high adherence to an adherent whose bonding is known as being extremely difficult, such as a polyolefin, and which, in the use in the formation of a pressure sensitive adhesive tape, ensures excellent applicability and curability.
It is still a further object of the present invention to provide a pressure sensitive adhesive tape wherein use is made of the above pressure sensitive adhesive composition, and to provide a process for producing the same.
The acrylic pressure sensitive adhesive composition of the present invention comprises:
(a) 5 to 75 parts by weight of an adherent polymer comprising (meth)acrylic ester component units as principal structural units and having a weight average molecular weight of 50,000 or more,
(b) 5 to 40 parts by weight of a tackifier resin comprising (meth)acrylic ester component units as principal structural units and having a weight average molecular weight of 20,000 or less, and
(c) 20 to 90 parts by weight of monomers whose principal component is a (meth)acrylic ester,
provided that the sum of component (a), component (b) and component (c) is 100 parts by weight,
wherein substantially no solvent is contained.
The process for producing a pressure sensitive adhesive tape according to the present invention comprises coating a support surface with a mixture of 100 parts by weight of an acrylic pressure sensitive adhesive composition and 0.01 to 3 parts by weight of a polymerization initiator at a thickness of 0.01 to 1.0 mm and polymerizing the mixture,
the above acrylic pressure sensitive adhesive composition comprising:
(a) 5 to 75 parts by weight of an adherent polymer comprising (meth)acrylic ester component units as principal structural units and having a weight average molecular weight of 50,000 or more,
(b) 5 to 40 parts by weight of a tackifier resin comprising (meth)acrylic ester component units as principal structural units and having a weight average molecular weight of 20,000 or less, and
(c) 20 to 90 parts by weight of monomers whose principal component is a (meth)acrylic ester,
provided that the sum of component (a), component (b) and component (c) is 100 parts by weight,
substantially no solvent contained in the acrylic pressure sensitive adhesive composition.
The first pressure sensitive adhesive tape of the present invention is one obtained by coating a support surface with a mixture of 100 parts by weight of an acrylic pressure sensitive adhesive composition and 0.01 to 3 parts by weight of a polymerization initiator at a thickness of 0.01 to 1.0 mm and polymerizing the mixture,
the above acrylic pressure sensitive adhesive composition comprising:
(a) 5 to 75 parts by weight of an adherent polymer comprising (meth)acrylic ester component units as principal structural units and having a weight average molecular weight of 50,000 or more,
(b) 5 to 40 parts by weight of a tackifier resin comprising (meth)acrylic ester component units as principal structural units and having a weight average molecular weight of 20,000 or less, and
(c) 20 to 90 parts by weight of monomers whose principal component is a (meth)acrylic ester,
provided that the sum of component (a), component (b) and component (c) is 100 parts by weight,
substantially no solvent contained in the acrylic pressure sensitive adhesive composition.
The acrylic pressure sensitive adhesive composition of the present invention is characterized by comprising an adherent polymer (a) of 50,000 or more weight average molecular weight having a low glass transition temperature and having tackiness at room temperature, a tackifier resin (b) with a glass transition temperature higher than room temperature which has no tackiness at room temperature but, upon being added to a pressure sensitive adhesive composition, can impart tackiness to the pressure sensitive adhesive composition, and a monomer (c). A pressure sensitive adhesive of high bond strength can be produced by, for example, adding a polymerization initiator to the acrylic pressure sensitive adhesive composition and polymerizing the mixture. In particular, the acrylic pressure sensitive adhesive composition of the present invention, when applied to, for example, a polyolefin whose bonding is considered to be difficult because of low affinity to pressure sensitive adhesives, it exhibits excellent pressure sensitive adhesive property.
A pressure sensitive adhesive tape can be produced by coating a support surface with the acrylic pressure sensitive adhesive composition of the present invention and polymerizing the acrylic pressure sensitive adhesive composition on the support. This acrylic pressure sensitive adhesive composition contains a tackifier resin of high glass transition temperature and low molecular weight. The addition of this tackifier resin causes the obtained pressure sensitive adhesive tape to have a peel strength greater than those of conventional pressure sensitive adhesive tapes. Further, this acrylic pressure sensitive adhesive composition does not contain any solvent, so that not only it is not needed to expel a solvent in the production of pressure sensitive adhesive tape to thereby enable producing a pressure sensitive adhesive tape with reduced operation but also environmental pollution by solvents can be avoided.
The second acrylic pressure sensitive adhesive tape of the present invention comprises a support and, superimposed on at least one surface thereof, a pressure sensitive adhesive layer,
the above pressure sensitive adhesive layer formed by coating the support surface with a pressure sensitive adhesive composition and photopolymerizing the pressure sensitive adhesive composition,
the above pressure sensitive adhesive composition comprising:
a partial polymerizate of monomers each having a polymerizable unsaturated bond whose principal component is an acrylic acid alkyl ester, the above partial polymerizate containing polymers of the monomers in an amount of 5% by weight or more,
a crosslinking agent, and
a photopolymerization initiator.
The process for producing the second acrylic pressure sensitive adhesive tape according to the present invention comprises coating at least one surface of a support with a pressure sensitive adhesive composition so as to form a coating layer and irradiating the coating layer with light so that the pressure sensitive adhesive composition is photopolymerized to thereby obtain the pressure sensitive adhesive layer superimposed on the support surface,
the above pressure sensitive adhesive composition comprising:
a partial polymerizate of monomers each having a polymerizable unsaturated bond whose principal component is an acrylic acid alkyl ester, the above partial polymerizate containing polymers of the monomers in an amount of 5% by weight or more,
a crosslinking agent, and
a photopolymerization initiator.
The above second acrylic pressure sensitive adhesive tape of the present invention exhibits highly excellent adhesive properties because it is produced by coating a support with a pressure sensitive adhesive comprising a specified partial polymerizate, a crosslinking agent and a photopolymerization initiator and photopolymerizing the pressure sensitive adhesive. In particular, a pressure sensitive adhesive tape of excellent thermal stability can be obtained by the use of a partial polymerizate prepared with the use of a specified thermal polymerization initiator.